1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile machine, and more particularly, to an electrophotographic image forming apparatus, and a control method therefor.
2. Discussion of the Background
In general, intermediate transfer-type image forming apparatuses primarily transfer toner images developed on image carriers (e.g., photoreceptors) onto an intermediate transfer member (e.g., intermediate transfer belt), ultimately obtaining finished images by secondarily transferring the toner images formed on the intermediate transfer belt onto sheet-like recording media (hereinafter also “sheets”).
In certain commonly used image forming apparatuses including multiple photoreceptors, the toner images are sequentially transferred onto the intermediate transfer belt and superimposed one on another thereon, forming a multicolor toner image on the intermediate member. Then, the multicolor toner image on the intermediate transfer member is transferred onto the sheet at once, that is, in a single operation.
In intermediate transfer-type image forming apparatuses, in order to reduce positional deviation and color deviation when operating at high speed, a material such as polyimide resin or polyamide-imide resin is used for the intermediate transfer belt because of its greater elasticity.
However, a drawback of such highly elastic intermediate transfer belts, is that when high-asperity sheets are used it is difficult to attain uniform images because the transfer electric field that is required to transfer the toner from the belt to the sheet is not constant in space but varies. More specifically, gaps are generated between the toner image on the intermediate transfer belt and concave portions of the sheet, thus weakening the transfer electric field compared to convex portions of the sheet. Consequently, it is difficult to reliably transfer the toner image onto the sheet, and white voids become noticeable. This is what is meant herein by the phrase “difficult to attain uniform images”.
Moreover, simply increasing a secondary transfer current or a secondary transfer voltage to strengthen the transfer electric field does not solve the problem because discharging occurs in the gap formed in the concave portion, and the polarity of the charge of the toner is reversed by the discharging. Thus, transfer efficiency is degraded, and white void becomes noticeable.
In particular, in image forming apparatuses that keep the secondary transfer current constant, that is, perform constant-current control, the balance among the amount of current flowing due to the movement (transfer) of the toner from the intermediate transfer belt to the sheet, the amount of discharge, and the current flowing directly to a non-image area (i.e., a blank area) of the sheet constantly changes in accordance with printing ratio, the strength of the charge of the toner, and/or the resistivity of the sheet. (Here, the term “printing ratio” refers to the ratio of the image area to the width of the sheet.) Failure to maintain that balance can result in faulty images.
In view of the foregoing, several approaches described below have been tried.
In one known method, for image forming apparatuses, in order to increase adhesion between the toner on the intermediate transfer belt and concave portions of high-asperity sheets, the pressure with which a secondary-transfer bias roller and a secondary-transfer facing roller press against each other is increased.
However, in this case, the pressure is biased toward the toner contacting the concave portion. At this time, non-electrostatic adhesion force among toner particles or between the toner particles and the intermediate transfer belt is increased, with the result that, in some instances, the toner cannot be transferred onto the sheet, in particular, in images consisting of text or thin lines.
In another approach, in order to solve the problem, the intermediate transfer belt is given a laminated two layer-structure, a core layer and an outer layer. The core layer is formed of a material having a higher elasticity and the outer layer is formed of a material having a lower elasticity.
However, in this case, because the respective layers in such a laminated intermediate transfer belt are bonded together with conductive adhesive, it can happen that resistivity is uneven, durability is reduced, and the cost of the intermediate transfer belt is increased.
In order to produce uniform images even on high-asperity sheets regardless of changes in the printing ratio, it is important that the maximum electric field be stably formed within a range that discharging does not occur in the concave portions of the sheets. Namely, it is required that the current applied to the rollers be used almost completely for the movement of the toner from the intermediate transfer belt to the sheet while preventing occurrence of unnecessary discharging.
However, as described above, when the secondary transfer current is kept constant, even when the printing ratio in a main scanning direction in the transfer rollers, that is, the total amount of the charge of the toner to be transferred changes, balance among the amount of current flowing due to movement of the toner from the intermediate transfer belt to the sheet, the amount of discharge, and the amount of current flowing directly to a non-image area of the sheet may be changed. Therefore, with these known approaches described above, consistently applying the current required to transfer the toner for various different images is impossible.
Therefore, in another known method, the secondary transfer current applied to the secondary transfer roller is not kept constant but is changed in accordance with image data. Compared to maintaining a constant secondary transfer current, contrast transfer efficiency can be obtained for various printing ratio of the image.
As another method, controlling the secondary transfer current in accordance with a detected primary transfer voltage has been proposed. In this method, the secondary transfer current is controlled in accordance with the detected primary transfer voltage, in particular the primary transfer voltage as it is affected by humidity and temperature.
However, in this known approach, because changes in the primary transfer voltage in a sub-scanning direction are not detected, the secondary transfer cannot performed in accordance with changes in the printing ratio in a sub-scanning direction caused by fluctuation in the sub-scanning direction.
As described above, in the image forming apparatus, when the secondary transfer current is simply controlled based on the image data, uniform images cannot be produced because of changes in the printing ratio. More specifically, because the amount of the toner adhering to the photoreceptor and the strength of the charge on the toner changes with fluctuations in humidity or temperature, and is affected also by deterioration of the photoreceptor or the developer, the printing ratio from the image data might differ from the printing ratio on the photoreceptor (area ratio of the toner image on the photoreceptor).
Accordingly, there is a need for a technology to attain uniform image transfer in the image forming apparatus even when the intermediate transfer member has great elasticity and the transferred material has large asperity, regardless of changes in the printing ratio.